Perhaps you should try some spelling variations:
"aberration theory" » "aberration test"
"operation theory" » "operations theory", "generation theory", "operation theatre"
"iteration theory" » "integration theory", "interaction theory", "vibration theory"
"operator theory" » "operators theory"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("operator theory") OR ((((("operation theory") OR ("aberration theory"))) OR ("iteration theory")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"operator theory\") or (((((\"operation theory\") OR (\"aberration theory\"))) or (\"iteration theory\"))))".
• Removing quotes may allow a broader search: (( operator theory ) or ((((( operation theory ) OR ( aberration theory ))) or ( iteration theory )))).
• Adding a wildcard symbol may retrieve word variants: (("operator theory") or ((((("operation theory") OR ("aberration theory"))) or ("iteration theory"))))*.